The present invention relates to a reaction vessel heated by helium under pressure, with reaction tubes filled with catalyst arranged within the vessel, the inlet and outlet pipes for feedstock and product being connected to the reaction tube head. Such reaction vessels with reaction tubes filled with catalyst are used for the steam reforming of hydrocarbons in an endothermic reaction. The heat required for the reaction is provided by helium heated to a high temperature outside of the reaction vessel, e.g. in a nuclear reactor. For reasons of safety, the reaction vessels, which are usually several in number, are arranged radially about the nuclear reactor and mounted in pre-stressed concrete together with the nuclear reactor. The reaction vessels are thus perpendicular to the outer surface of the concrete block and only accessible from above through the covers.
In such reaction vessel, it is necessary to provide separate inlet and outlet pipes for the feedstock and the product for each reaction tube in a reaction vessel so that in case of damage to any reaction tube, such tube can be isolated by blocking off the inlet and outlet pipes outside the reaction vessel, thus obviating the need to shut down the reaction vessel or the entire installation.
It is known to pass the individual inlet and outlet pipes either separately or in groups through the cover of the reaction vessel in order to meet the above requirement. A large number of design problems arise however, since over 100 reaction tubes are arranged in such a reaction vessel and the gas temperatures of the feedstock and the product range from about 600.degree. K to 1100.degree. K.
Thus, it has already been proposed to reduce the product gas temperature by passing the product from the bottom of the reaction tube back to the top of the tube via a helical tube installed within the reaction tube during which process the product undergoes a reduction of temperature by heat exchange against the inflowing feedstock, i.e. hydrocarbons or the reacting gas mixture. In this manner one succeeds in reducing the temperature of the product gas to about 880.degree. K before it reaches the outlet pipe. A temperature of about 880.degree. K is, however, still very high, the design temperature of the cover thus being unfavorably high also and giving rise to unpredictable thermal and mechanical stresses in the cover during start-up and operation. In addition, further unfavorable conditions arise, such as a structural weakening of the cover brought about by the passage of numerous pipes, as well as difficulties in assembly, dismantling and accessibility. If two reaction vessel covers are necessary for reasons of safety, then the above-mentioned disadvantages are even more evident.